The average age of a teacher and three students is $20$ years. If all the students are of the same age and the difference between the teacher's age and the age of one of the students is $20$ years,find the teacher's age in years.

If in an examination different weights are assigned to different subjects: Physics $(2)$,Chemistry $(1)$,English $(1)$,and Mathematics $(2)$. If a student scored $60$ in Physics,$70$ in Chemistry,$70$ in English,and $80$ in Mathematics,then his weighted $A.M.$ is:

Which of the following,in the case of discrete data,is not equal to the median?

If the mean of five observations $x, x + 2, x + 4, x + 6$,and $x + 8$ is $11$,what is the mean of the last three observations?

The following data gives the distribution of height of students. The median of the distribution is:

Height (in $cm$)	$160$	$150$	$152$	$161$	$156$	$154$	$155$
No of students	$12$	$8$	$4$	$4$	$3$	$3$	$7$

The mean of a set of numbers is $\bar{x}$. If each number is multiplied by $\lambda$,then the mean of the new set is